The present invention relates to a self-frequency-doubler laser element having a laser oscillation function in emitting a laser excitation beam and a wavelength conversion function in converting the fundamental harmonic that has been generated by the laser excitation beam to a second higher harmonic having half the wavelength of that of the fundamental harmonic.
Heretofore, attempts have been made to reduce a laser beam to having a short wavelength and there have been proposed some optical wavelength conversion elements for converting the wavelength of a laser beam for the purpose.
For example, an optical wavelength conversion element of a bulk crystal type has been introduced in "The Fundamentals of Optoelectronics" by A. Yarlv, translated by Kunio Tada and Takeshi Kamiya, pp 200 to 204, Published by Maruzen Co., Ltd. However, it is essential for second higher harmonics produced in a nonlinear optical material to match and strengthen each other, that is, the phase matching condition needs establishing in the optical wavelength conversion element of the bulk crystal type. Therefore, the birefringence of the crystal has to be utilized and there still arises a problem in that materials having no birefringence properties or few of them are not directly utilizable even though they are nonlinear at a molecular level.
As an optical wavelength conversion element capable of solving the above problem, an optical wavelength conversion element of a three-dimensional optical waveguide type has been introduced in, for example, "Extended Abstracts, Physical Concepts of Materials for Novel Optoelectronic Device Applications," by O. Sugiura et al., SPIE, Vol. 1361, p 599 (1990). Typical second higher harmonic phase matching methods making use of The three-dimensional optical waveguide are known to include (1) a method of matching mode-to-mode phases of a fundamental harmonic and a second higher harmonic under control over film thickness, and (2) a method of matching phases in between the waveguide mode of a fundamental harmonic and a radiation mode toward the ground of a second higher harmonic. The method (1) is said to be particularly promising because an output proportional to the square of the propagation length is obtainable.
Studies are enthusiastically being made of optical wavelength conversion elements which employ organic nonlinear optical materials whose nonlinearity and response speed are greater than those of inorganic nonlinear optical materials. Moreover, attempts have also been made to combine such an optical wavelength conversion element with a laser so as to develop a laser in a visible range.
If, however, an optical system is fabricated with an Nd.sup.3+ YAG laser intended for a fundamental harmonic, the laser tends to become large in shape and consequently the system may not be made compact since a light source such as a xenon lamp is used for excitation light. As a result, it is attempted to make a system compact by employing a semiconductor laser for a fundamental harmonic. When the semiconductor laser is used for an optical system, an internal resonance laser will fall under the category of use. However, organic crystals lack transparency as their processability is bad at the present technological level and this poses a problem of failure in sufficient power.